Liquid crystals are applied to display media in which the reversible movement of liquid crystal molecules is made use of, such as display devices typically of TN or STN mode, Besides the application to display media, liquid crystals have been studied for applicability to optical elements, such as a retardation film, a polarizer, a polarizing prism, a luminance-improving film, a low pass filter, and various optical filters, taking advantage of their anisotropy in physical properties, such as refractive index, dielectric constant, magnetic susceptibility, elastic modulus, and thermal expansion coefficient, as well as their alignment properties.
The above-described optical elements are obtained by, for example, uniformly aligning the molecules of a liquid crystal compound having a polymerizable functional group or a polymerizable composition containing the liquid crystal compound into a liquid crystal phase and irradiating the compound or the composition being in the liquid crystal phase with energy rays, such as ultraviolet rays, to cause photopolymerization. It is required to fix the aligned state of the liquid crystal compound uniformly and semi-permanently.
A polymer can be obtained by applying a polymerizable composition to a substrate and polymerizing the coating film. To obtain a polymer film with a uniform thickness, it is preferred to use the polymerizable composition in the form of a solution in a solvent. Should the polymerizable composition be insoluble in a solvent, it is very difficult to control the molecular alignment of the liquid crystal compound. Hence, it is required for the polymerizable compound or the polymerizable composition containing it to have good solubility in a solvent.
A polymerizable compound having a (meth)acrylic group as a polymerizable functional group exhibits high polymerization reactivity, and the resulting polymer has high transparency and has therefore been extensively studied for use as an optical element as described above.
Bifunctional polymerizable compounds having two (meth)acrylic groups as polymerizable functional groups are proposed, e.g., in Patent Document 1 to 7. However, using these polymerizable compounds can result in problems, such as precipitation of crystals on polymerizing or difficulty of controlling uniform alignment.
Patent Document 1: JP 11-16534A
Patent Document 2: JP 2002-145830A
Patent Document 3: JP 2003-315553A
Patent Document 4: JP 2005-263789A
Patent Document 5: JP 11-130729A
Patent Document 6: JP 2005-309255A
Patent Document 7: WO2006/049111